Fluoropolymer

ABSTRACT

A fluoropolymer having a melting point of at least 160° C. is provided wherein the fluoropolymer is derived from tetrafluoroethylene as the major component, one or more perfluoro vinyl ethers as the minor component, the perfluoro vinyl ether having the formula CF 2 ═CFO(CF 2 ) n OR f  wherein n is from 2 to 6 and R f  is a perfluoroalkyl group having 1 to 6 carbon atoms, and optionally a perfluoro alkyl vinyl ether having from 1 to 5 carbon atoms in the alkyl radical.

[0001] The present invention relates to fluoropolymers which have amelting point of at least 160° C. and which are prepared fromtetrafluoroethylene (TFE), from perfluoro vinyl ethers, and also, whereappropriate, from conventional olefinic comonomers, such ashexafluoropropylene (HFP), or olefins which contain hydrogen or containchlorine.

[0002] These fluoropolymers are either thermoplastic or are notprocessable in the melt despite their possession of a melting point.

[0003] Typical representatives of thermoplastically processable TFEcopolymers are copolymers made from TFE and from perfluoro (alkyl vinyl)ethers (PAVE) having from 1 to 3 carbon atoms in the alkyl radical, inparticular perfluoro (n-propyl vinyl) ether (PPVE-1) (U.S. Pat. No.3,132,123). Such copolymers are commercially available under thedesignation “PFA”. Starting from a comonomer content of about 2% byweight PAVE, these partially crystalline copolymers have outstandingtechnical performance, for example extraordinary chemical stabilitycombined with high service temperatures. The previously known and usedPFA products are copolymers made from TFE and from PPVE-1 (U.S. Pat. No.3,635,926) as well as terpolymers made from TFE/PPVE-1 and HFP (DE-A-2639 109). Copolymer products which contain perfluoro ethyl vinyl ether(WO-A-97/07147) or perfluoro methyl vinyl ether (U.S. Pat. No.4,864,006) in place of PPVE-1 are also to be found in the literature.However, these copolymers have low flexural fatigue strength and lowflexibility, and therefore have little suitability for corrugated tubingapplications.

[0004] Another commonly used and thermoplastically processable materialis fluorinated ethylene-propylene copolymer (FEP) (U.S. Pat. No.2,946,763, U.S. Pat. No. 2,955,099, U.S. Pat. No. 3,085,083). Thesecopolymers are not highly resistant with regard to thermal or chemicalstress cracking, or to flexural fatigue properties. These properties areimproved by incorporating PPVE-1 (DE-A-27 10 501) into the FEP. However,a relatively large amount of PPVE-1 is needed in order to achieve arational property profile, and this is very costly (EP-A-75 312). Theamount can be reduced by using relatively long-chain vinyl ethers, inparticular by incorporating perfluoro 2-propoxypropyl vinyl ether(PPVE-2). PPVE-2 is only cost-effectively usable in non-aqueouspolymerization (EP-A-75 312). Otherwise the polymerization times are toolong to be cost-effective.

[0005] Fluorinated thermoplastics tend to degrade thermally whenprocessed. The thermal degradation takes place predominantly via thethermally unstable end groups formed during the polymerization, i.e.from the end of the chain. The mechanism of this degradation has beendescribed in some detail in “Modern Fluoropolymers”, John Wiley & Sons,1997, and in K. Hintzer and G. Löhr, ‘Melt ProcessableTetrafluorethylene-Perfluoropropylvinyl ether copolymers (PFA)’, page223. During the thermal degradation corrosive gases are produced andconsiderably impair the quality of the final product via metalcontamination or bubble formation, and can corrode tooling andprocessing machinery. The effect naturally increases as molecular weightfalls (a lower melt viscosity). The degradation can be substantiallysuppressed by converting the thermally unstable end groups into stableCF₃ end groups by postfluorination of agglomerate or pelletized melt,for example as described in U.S. Pat. No. 4,743,658 and DE-A-19 01 872.To achieve particularly pure products, a stationary bed of agglomeratemay be fluorinated as in U.S. Patent Application No. 60/208,626 datedJun. 1, 2000. For fast extrusion, in accordance with WO-A-99/41313preference is given to bi- or multimodal molecular weight distributions.

[0006] Polytetrafluoroethylene (PTFE) was the first commerciallyavailable perfluorinated fluoropolymer, and its property profiles havebeen continually improved in recent decades. However, its very highmolecular weight makes it processable only by sinter techniques or otherspecialized processing methods, such as paste extrusion. The polymerchains have a linear structure, and this can be highly disadvantageousin applications, since the polymer chains slide over one another when aforce is applied. This causes outflow of the PTFE material, a phenomenonknown as “cold flow” which is problematic in gasket applications. Thisproblem has been reduced, but not solved, by what is known asmodification, i.e. incorporation of comonomers, such as PPVE-1, inamounts of from 0.01 to 0.1% by weight (U.S. Pat. No. 3,855,191). Therestill remains a need, therefore, to suppress “cold flow” and also toincrease weldability.

[0007] U.S. Pat. No. 3,817,960 discloses low and high molecular weightcopolymers of TFE and a perfluorovinyl ether of the formulaCF₃(CF₂O)_(n)CF₂CF₂OCF═CF₂. The low molecular weight copolymers aretaught useful as thermally stable oils and greases and the highmolecular weight can be used for making molded articles.

[0008] U.S. Pat. No. 4,920,170 and U.S. Pat. No. 5,401,818 discloseelastomeric polymers that include certain units derived from fluorovinylethers. Typical applications taught for these polymers include moldedarticles such as gaskets, O-rings, flange seals etc.

[0009] It is an object of the present invention to provide athermoplastically melt processable material which has high flexuralfatigue strength, high flexibility and low susceptibility to stresscracking, and secondly to reduce further the “cold flow” of TFE polymerwhich can be processed through sintering. The polymerization shouldpreferably be carried out in an aqueous medium.

[0010] To solve all of these problems, units derived from one or moreperfluorovinyl ether comonomers as defined below are incorporated in thefluoropolymer. The perfluorovinyl ether comonomer has a relatively longand relatively mobile, linear side chain, and can conveniently beincorporated into the polymer in an aqueous emulsion polymerization.

[0011] In particular, the present invention relates to fluoropolymersthat have a melting point of at least 160° C. and that comprise from 60to 99.99% by weight of units derived from tetrafluoroethylene, from 0 to30% by weight of units derived from hexafluoropropylene, from 0 to 10%by weight of units derived from other olefinic fluorinated ornon-fluorinated monomers, from 0 to 35% by weight of units derived fromperfluoro alkyl vinyl ether having from 1 to 5 carbon atoms in the alkylradical, and from 0.01 to 35% by weight of units derived from one ormore perfluoro vinyl ethers of the formula:

CF₂═CF[O(CF₂)_(n)]_(m)(OCF₂)_(x)OR_(f)  [I]

[0012] where

[0013] n is an integer from 2 to 6,

[0014] m is an integer from 1 to 3,

[0015] x is an integer from 0 to 4, and

[0016] R_(f) is a perfluoroalkyl group having from 1 to 6 carbon atoms.

[0017] In a particular embodiment of the invention, the fluoropolymerhas between 65% by weight and 99.99% by weight of units derived fromtetrafluoroethylene. In a further embodiment, the fluoropolymer hasbetween 70% by weight and 99.99% by weight of units derived fromtetrafluoroethylene and between 0.01% by weight and 30% by weight ofunits derived from perfluorovinyl ethers of formula [I]. Typically, theamount of units of perfluorovinyl ethers of formula [I] will be between0.1% by weight and 15% by weight, preferably between 0.3% by weight and10% by weight. It will be understood that if the fluoropolymer is acopolymer of only TFE and the perfluorovinyl ether(s) of formula [1],the remaining units will be TFE. If the fluoropolymer is a copolymer ofTFE, the perfluorovinyl ether(s) of formula [I] and further comonomersother than the perfluorovinyl ether(s) of formula [1], the amounts ofTFE and the further comonomers should be as set forth above.

[0018] In one embodiment of the invention, the fluoropolymer isthermoplastically melt processable and can thus readily be used to makeextruded articles. By the term “thermoplastically melt processable” ismeant that the fluoropolymer has a sufficiently low melt viscosity suchthat the melt of the fluoropolymer can be processed by typicallyavailable extrusion equipment. Generally, the melt flow index at 372° C.measured as set forth in the examples, should be more than 0, preferablyat least 0.1 so as to achieve a thermoplastically melt processablefluoropolymer.

[0019] Thus, in a particular aspect, the invention also relates to amethod of making an extruded article with the aforementionedfluoropolymers and to extruded articles such as films, tubes, hoses andcables having the fluoropolymer as an insulating medium.

[0020] According to the invention it has been found that the use of evensmall amounts of units derived from the comonomer of formula (I), whoseside chain is linear, long and flexible markedly improves flexuralfatigue strength, flexibility, and chemical and thermal stress crackingperformance. Like PPVE-1, but unlike the branched material PPVE-2,perfluoro 3-methoxy-n-propyl vinyl ether (PMPVE) which has the followingformula

F₂C═CF—O—CF₂—CF₂—CF₂—O—CF₃

[0021] or perfluoro 2-methoxyethyl vinyl ether (PMEVE) which has theformula

F₂C═CF—O—CF₂—C F₂—O—CF₃

[0022] in particular can be copolymerized with TFE in aqueous emulsionpolymerization. In comparison with PPVE-1, the side chain in PMPVE andPMEVE is longer, and in comparison with PPVE-2 the side chain in PMPVEand PMEVE is more mobile. Used as “modifiers” in PTFE, PMPVE and PMEVEcause a substantially greater reduction in the “cold flow” of suchproducts, and to an improvement in their weldability. The unbranchedside chain is significant. Other preferred comonomers with a longmobile, linear side chain are:

[0023] F₂C═CF—O—CF₂—CF₂—O—CF₂—CF₃,

[0024] F₂C═CF—O—CF₂—CF₂—CF₂—O—CF₂—CF₃,

[0025] which can be prepared as in U.S. patent application Ser. No.09/470,497, filed on Dec. 22, 1999,

[0026] F₂C═CF—O—CF₂—CF₂—O—CF₂—O—CF₃,

[0027] F₂C═CF—O—CF₂—CF₂—(O—CF₂)₂—O—CF₃,

[0028] F₂C═CF—O—CF₂—CF₂—(O—CF₂)₃—O—CF₃,

[0029] F₂C═CF—O—CF₂—CF₂—(O—CF₂)₄—O—CF₃,

[0030] which can be prepared as in DE-A-22 15 401 or U.S. Pat. No.3,692,843.

[0031] The long mobile, linear side chains improve the property profileof fluorinated thermoplastics, and also of PTFE products modified withthe comonomer of formula [1).

[0032] Compared with a conventional PFA product with the same meltingpoint, a copolymer of the invention is more flexible, i.e. has lowerstiffness, and the product has a smaller spherolite structure. This canbe seen from the markedly lower modulus of elasticity (119 MPa) of acopolymer of TFE and PMPVE, compared with a conventional PFA with amodulus of elasticity of 137 MPa at 120° C. Flexural fatigue strengthtests revealed a marked improvement in this property in thefluoropolymer in comparison with a PFA. Indeed, a PMPVE containingpolymer has a flexural fatigue strength 20% higher than that of a knownPFA product. There is a reduction by a factor of 10 in spherolitestructure for the same melting point. Its average value is 3 μm. Thisdirectly affects the surface structure of hose, specifically giving avery low surface roughness on the internal wall. This has hitherto beenpossible only by way of an additional nucleation (U.S. Pat. No.5,473,018) or as in the German Patent Application 199 64 006.8 of Dec.30, 1999, which is not a prior publication, by way of the additionalincorporation of PPVE-2 into the PFA. Surface roughness is anincreasingly important factor in the supply of ultrahigh-purity media,since lower roughness reduces cleaning cost, in particular reducingconditioning times, and produces less microbacterial growth.

[0033] A FEP copolymer modified with a perfluorovinyl ether monomer offormula [I], for example an FEP polymer modified with PMPVE is moreresistant than an unmodified FEP or a PPVE-1-modified FEP grade to mediawhich cause stress cracking. This difference can be illustrated usingthe elongation at 200° C. An FEP product modified with PMPVE has 600%higher elongation at 200° C. than a comparable FEP copolymer. This is ofparticular interest for the use of FEP materials as electricalinsulation in what are known as “Local Area Networks” (LAN). Here,besides non-combustibility and good dielectric properties, theinsulation is also required to have resistance to high temperatures. Itis possible to achieve markedly better values for the mechanical data athigh temperatures with a relatively small content of “modifier”. Indeed,0.1 mol % of PMPVE in the FEP gives an elongation at break at 200° C.which is higher than with 0.25 mol % of PPVE-1 in the FEP.

[0034] Accordingly, in a particular embodiment, the present inventionprovides a copolymer having a melting point of at least 240° C., havingfrom 80 to 99.99% by weight of units derived from tetrafluoroethylene,from 1% to 15% by weight, of units derived from hexafluoropropylene,from 0 to 10% by weight of units derived from other olefinic fluorinatedor non-fluorinated monomers, from 0 to 18.99% by weight of units derivedfrom perfluoro alkyl vinyl ether having from 1 to 5 carbon atoms in thealkyl radical, and from 0.01 to 19% by weight of units derived from oneor more perfluoro vinyl ethers of the formula:

CF₂═CF[O(CF₂)_(n)]_(m)(OCF₂)_(x)OR_(f)  [I]

[0035] where

[0036] n is an integer from 2 to 6,

[0037] m is an integer from 1 to 3,

[0038] x is an integer from 0 to 4, and

[0039] R_(f) is a perfluoroalkyl group having from 1 to 6 carbon atoms.

[0040] A terpolymer composed of TFE, HFP and a perfluorovinyl ether offormula (1], e.g. PMPVE, and having a melting point of 285° C. or morehas better flexural fatigue strengths than a corresponding terpolymermade from TFE/HFP and PPVE-1. Indeed, incorporating the same weight ofether comonomer units of formula [I] gives a terpolymer product whoseflexural fatigue strength is 35% higher using PMPVE than can be achievedusing PPVE-1. The effect of the long and mobile side chain of PMPVE isclearly seen in the lower modulus of elasticity, only 114 MPa at 120°C., compared with 123 MPa for a terpolymer comprising PPVE-1. This novelmaterial is easy to process and gives a very smooth inner surface whenproducing hose. In addition, this material has high transparency.

[0041] At increased proportions of comonomers HFP and comonomersaccording to formula [1], e.g. PMPVE, products can be prepared whichhave melting points of from 160 to 220° C. and have excellentsuitability as processing aids for blown film production, or as acoating material or insulating material in gasoline hose. Thesematerials have high chemicals resistance, low permeability and goodflexibility, together with favorable processing conditions. Theseproperties are preferably achieved using a perfluorinated polymer chainand a high proportion of “modifier”.

[0042] To improve chemical and physical binding to hydrogen-containingpolymers, it is preferable to include up to 10% by weight of ahydrogen-containing comonomer such as for example ethylene, propylene orvinylidene fluoride. This binding is of interest in producing gasolinehose, and there is only an insignificant increase here in thepermeability of the fluoropolymer. The low-melting products producedaccording to the invention have markedly better mechanical propertiesthan the known low-melting FEP grades (EP-A-656 912 and EP-A-731 814).

[0043] To give the fluoropolymer better resistance to thermaldegradation, and also to improve their performance and processingproperties for certain relevant applications, the thermally unstable endgroups produced by the polymerization may be converted into thermallystable —CF₃ end groups by reaction with fluorine.

[0044] Modification of PTFE with PMPVE rather than PPVE-1 or HFPincreases long-term flexural strength and gives a smoother surface. Thesignificantly lower deformation under load (“cold flow”) should beemphasized.

[0045] Deformation under load plays a decisive part in the use of PTFEas a gasket material, its use being preferred because of its chemicalsresistance and heat resistance. If the deformation is excessive, thePTFE material can be expelled, and this in turn causes leaks. A PTFEmaterial with low “cold flow” ensures markedly greater reliability andserviceability in these applications.

[0046] In tensile creep tests at room temperature with tensile stressesof 3 MPa, 5 MPa and 7 MPa, this PMPVE-modified PTFE material has anelongation after 100 h of only 50% of that of PPVE-1-modified PTFEgrades, and only 25% of that of unmodified PTFE grades. The “cold flow”is therefore drastically reduced in the modified PTFE material, and thisis advantageous in gasket applications. In addition, a PMPVE-modifiedPTFE material can be welded and thermoformed, and is thereforeparticularly well suited to inner linings in pipes or in containers.

[0047] The PTFE modified material for use in a gasket application or foruse as inner lining in pipes or containers is typically a copolymer ofTFE and one or more comonomers of formula [I] above wherein the amountof TFE is between 98.5% by weight and 99.99% by weight and whereby theamount of the comonomers according to formula [I] is between 0.01% byweight and 1.5% by weight. Such a PTFE modified material will typicallyhave a melting point of at least 315° C. and may not be melt processablebut will be processable through sinter techniques or paste extrusion.

[0048] The copolymers according to the invention can convenientlyprepared through aqueous emulsion polymerization of appropriate amountsof the composing monomers or through suspension. The obtained polymerdispersion after aqueous emulsion polymerization can be used as such orif higher solids are desired, can be upconcentrated. Alternatively, thedispersion may be agglomerated to produce the polymer in agglomerateform. Agglomerates will typically have an average size of 1 to 5 mm. Ifthe agglomerates obtained from agglomerating the dispersion are toosmall, it may be desirable to compact the agglomerate to produce acompacted agglomerate which will typically have an average size of 1 to10 mm. Still further, if the polymer is melt processible, the polymermay be melted, extruded and cut into granulates of a desired size. Thelatter may be called melt granulate.

[0049] Test Methods:

[0050] The content of perfluorinated comonomers (U.S. Pat. No.4,029,868, U.S. Pat. No. 4,552,925) and the number of end groups(EP-A-226 668, U.S. Pat. No. 3,085,083) are determined by IRspectroscopy, using a Nicolet Magna 560 FTIR. The total of the endgroups is calculated from the isolated and bonded COOH, CONH₂ and COFgroups. PMPVE is likewise determined via the IR spectrum. For this, thequotient calculated from the signals at wavelengths 998/2365 cm⁻¹ isconverted using the factor 2.6 to obtain the proportion of PMPVE byweight.

[0051] The melt index (melt flow index, MFI) gives the amount of a meltin grams per 10 min which is extruded from a feed cylinder through a dieunder a piston loaded with weights. The dimensions of die, piston, feedcylinder and weights have been standardized (DIN 53735, ASTM D-1238).All of the MFIs mentioned here were determined using a 2.1 mm die with alength of 8 mm, for an applied weight of 5 kg and a temperature of 372°C.

[0052] Flexural fatigue strength tests are carried out on films of 1.0mm thickness. The device used here is model 956, No. 102 from Frank,year of construction 1967. The film strips required to determineflexural fatigue strength are 15 mm wide and have a minimum length of100 mm. For the test, adhesive strips are used to hold a piece of filmof approximately DIN A5 size onto the drum of a film cutter, adraw-knife system is put in place and the cutting drum is rotated tomanufacture strips at the given knife separation. The film strips areclamped into the screw clamps of the flexural fatigue machine, andloaded with an attached weight of 1529.6 g. The freely suspended filmstrips are flexed in both directions to an angle of 90° at the clampingsystem, with a flexing frequency of 250 double-flexures per minute,until fracture occurs. The number of double-flexures here is recorded bya counter located above the tester. The figure given is the number ofdouble-flexures before the film fractures. The flexural fatigue strengthof a material is calculated as the average value of the number ofdouble-flexures from the three measurement locations available on eachoccasion.

[0053] The spherolites were measured on pressure-sintered plaques of 2mm thickness, produced within a period of 30 min at 360° C. To this end,microtome sections were prepared from these pressure-sintered plaquesand studied by optical microscopy.

[0054] The tensile creep test to DIN 53444 serves to determinedeformation and strength performance. The test specimens used have athickness of 1 mm and are as described for test specimen No.5 in DIN53455, but their width is only 12.55 mm (UL fire test specimen) ratherthan 15 mm as in the standard. The free clamped length was 100 mm. Thetests were carried out at room temperature (23° C.) on a Zwick 1445universal testing machine, using a prescribed stress. The elongation inpercent was measured after 100 h.

[0055] The complex modulus of elasticity was determined on testspecimens stamped out from pressure-sintered plaques and having athickness of 1 mm, a length of 30 mm and a width of 6 mm. Adynamic-mechanical-thermal spectrometer from Gabo was used for thetests, in the temperature range from −100 to 250° C.

[0056] The tensile test for determining elongation at break at 200° C.was carried out on a Zwick universal testing machine. The test wascarried out to DIN 53455 on dumbbell specimens to ASTM D 1708, stampedout from pressure-sintered plaques.

[0057] The examples below describe the invention in greater detail.Percentages and ratios given are based on weight unless otherwisestated.

EXAMPLE 1

[0058] 25 l of deionized water are charged to a polymerization reactorwith a total volume of 40 l, provided with an impeller stirrer. Thereactor is sealed, and the atmospheric oxygen removed by cycles ofevacuation and nitrogen-flushing, and the vessel is heated to 63° C.After evacuation, 122 g of ammonium perfluorooctanoate in the form of a30% strength solution are added to the vessel. 180 g of PMPVE are thenpumped in. TFE is then set, with stirring, until the total pressure hasreached 13.0 bar. 19 g of methylene chloride are then added into thevessel. The polymerization is initiated by pumping in 2 g of ammoniumperoxodisulfate (APS), dissolved in 100 ml of deionized water. As soonas the pressure starts to fall, TFE and PMPVE are supplemented byintroducing the gases in a feed ratio PMPVE/TFE of 0.04, so that thetotal pressure of 13.0 bar is maintained. The heat generated isdissipated by cooling the vessel wall, keeping the temperature constantat 63° C. After a total of 7.2 kg of TFE has been fed into the reactor,the monomer feed is interrupted, the pressure in the reactor isreleased, and it is flushed several times with N₂. This gives acopolymer which has a PMPVE content of 3.6%, a melting point of 312° C.and an MFI of 1.8.

[0059] 4 kg of product are charged to a 5 l stainless steel reactor, andnitrogen at 130° C. is used to heat the reactor and to flush the sameuntil no air is present. A mixture of 20% of fluorine and 80% ofnitrogen is then passed into the reactor for 5×30 min. After thefluorination, the reactor is nitrogen-flushed until a fluorine sensor(0.01 ppm) no longer indicates fluorine. The resultant fluorinatedproduct still has 5 thermally unstable end groups.

[0060] Cycles Modulus of elasticity at Spherolite diameter in MFIModulus of elasticity at Spherolite (372/5) Cycles 120° C. (MPa)diameter in μm Example 1 1.8 21780 119 3

COMPARATIVE EXAMPLE 1

[0061] The polymerization is carried out as described in example 1,except that on this occasion PPVE-1 is utilized instead of PMPVE, with aPPVE-1/TFE feed ratio of 0.042. This gives a copolymer which has aPPVE-1 content of 4.0%, a melting point of 308° C. and an MFI of 1.9.The resultant product is also subjected to fluorination as described inexample 1 in such a way that 4 thermally unstable end groups are stillpresent after the fluorination. MFI Modulus of elasticity Spherolite(372/5) Cycles at 120° C. (MPa) diameter in μm Comparative 1.9 16641 13724 example 1

[0062] Compared with a PFA product from comparative example 1, acopolymer product from example 1, prepared using PMPVE, has higherflexural fatigue strength, lower modulus of elasticity and smallspherolites. It is therefore possible to prepare a flexible,thermoplastic, perfluorinated copolymer which is superior toconventional PFA products in terms of flexibility.

EXAMPLE 2

[0063] 25 l of deionized water are charged to a polymerization reactorwith a total volume of 40 l, provided with an impeller stirrer. Thereactor is sealed, and the atmospheric oxygen removed by cycles ofevacuation and nitrogen-flushing, and the vessel is heated to 70° C.After evacuation, 240 g of ammonium perfluorooctanoate in the form of a30% strength solution are added to the vessel. 90 g of PMPVE are thenpumped in. 0.13 bar of ethane is then added into the vessel, withstirring. TFE is then immediately fed until the pressure reaches 9 bar,and HFP is then applied under pressure until the total pressure of 17.0bar has been reached. The polymerization is initiated by pumping in 8 gof APS, dissolved in 100 ml of deionized water. As soon as the pressurestarts to fall, TFE, PMPVE and HFP are supplemented by introducing thegases in a feed ratio PMPVE/TFE of 0.02 and an HFP/TFE feed ratio of0.05, so that the total pressure of 17.0 bar is maintained. The heatgenerated is dissipated by cooling the vessel wall, keeping thetemperature constant at 70° C. After a total of 10 kg of TFE has beenfed into the reactor, the monomer feed is interrupted, the pressure inthe reactor is released, and it is flushed several times with N₂. Theresultant terpolymer has an HFP content of 5.2%, a PMPVE content of1.8%, a melting point of 287° C. and an MFI (372/5) of 10.1. MFI Modulusof elasticity (372/5) Cycles at 120° C. (MPa) Example 2 10.1 2980 114

COMPARATIVE EXAMPLE 2

[0064] The polymerization is carried out as described in example 2,except that on this occasion PPVE-1 is utilized instead of PMPVE, with aPPVE-1/TFE feed ratio of 0.02. This gives 7.0 kg of a copolymer whichhas a PPVE-1 content of 2.0%, an HFP content of 5.0%, a melting point of286° C. and an MFI of 8.0. MFI Modulus of elasticity (372/5) Cycles at120° C. (MPa) Comparative Example 2 8.0 2150 123

[0065] When compared with a terpolymer from comparative example 2,prepared using PPVE-1, a terpolymer from example 2, prepared usingPMPVE, gives a more flexible product with higher flexural fatiguestrength and lower modulus of elasticity.

EXAMPLE 3

[0066] 25 l of deionized water are charged to a polymerization reactorwith a total volume of 40 l, provided with an impeller stirrer. Thereactor is sealed, and the atmospheric oxygen removed by cycles ofevacuation and nitrogen-flushing, and the vessel is heated to 70° C.After evacuation, 240 g of ammonium perfluorooctanoate in the form of a30% strength solution are added to the vessel. TFE is then fed until thepressure reaches 6.5 bar, followed by application of HFP under pressureuntil the total pressure of 17.0 bar has been reached. Thepolymerization is initiated by pumping in 65 g of APS, dissolved in 100ml of deionized water. As soon as the pressure starts to fall, TFE,PMPVE and HFP are supplemented by introducing the gases in a feed ratioPMPVE/TFE of 0.01 and an HFP/TFE feed ratio of 0.1 1, so that the totalpressure of 17.0 bar is maintained. The heat generated is dissipated bycooling the vessel wall, keeping the temperature constant at 70° C.After a total of 10 kg of TFE has been fed into the reactor, the monomerfeed is interrupted, the pressure in the reactor is released, and it isflushed several times with N₂. The resultant terpolymer has an HFPcontent of 13.1%, an PMPVE content of 0.6%, a melting point of 260° C.and an MFI (372/5) of 28.

COMPARATIVE EXAMPLE 3

[0067] The polymerization is carried out as described in example 3, butthis time only using HFP, with an HFP/TFE feed ratio of 0.13. This givesa copolymer which has an HFP content of 14.5%, a melting point of 255°C. and an MFI of 27.0.

COMPARATIVE EXAMPLE 4

[0068] The polymerization is carried out as described in example 3,except that this time PPVE-1 is utilized instead of PMPVE, with aPPVE-1/TFE feed ratio of 0.01. This gives a copolymer which has a PPVE-1content of 1.0%, an HFP content of 13.0%, a melting point of 259° C. andan MFI of 25.

[0069] The incorporation of PMPVE into an FEP product as in example 3gives an improvement of about 600% in elongation at break at 200° C. incomparison with an unmodified FEP product from comparative example 3.Marked advantages when using PMPVE are also seen in comparison withPPVE-1 modified FEP product from comparative example 4. Flexural fatiguestrength has been increased by about 30% when PMPVE rather than PPVE-1has been used as “modifier”. TABLE 1 (Comparison of PMPVE-andPPVE-1-modified FEP products, and comparison with a standard FEPproduct) MFI Elongation at break at (372/5) Cycles 200° C. (%) Example 328 760 260% Comparative 25 490  41% Example 3 Comparative 27 570 190%Example 4

EXAMPLE 4

[0070] 25 l of deionized water are charged to a polymerization reactorwith a total volume of 40 l, provided with an impeller stirrer. Thereactor is sealed, and the atmospheric oxygen removed by cycles ofevacuation and nitrogen-flushing, and the vessel is heated to 70° C.After evacuation, 600 g of ammonium perfluorooctanoate in the form of a30% strength solution are added to the vessel. 300 g of PMPVE, 12 bar ofHFP and 5 bar of TFE are then fed, with stirring, so that the totalpressure of 17.0 bar is achieved. The polymerization is initiated bypumping in 20 g of APS, dissolved in 100 ml of deionized water. As soonas the pressure starts to fall, TFE, PMPVE and HFP are supplemented byintroducing the gases in a feed ratio PMPVE/TFE of 0.06 and an HFP/TFEfeed ratio of 0.15, so that the total pressure of 17.0 bar ismaintained. The heat generated is dissipated by cooling the vessel wall,keeping the temperature constant at 70° C. After a total of 10 kg of TFEhas been fed into the reactor, the monomer feed is interrupted, thepressure in the reactor is released, and it is flushed several timeswith N₂. The resultant terpolymer has an HFP content of 17%, a PMPVEcontent of 6.5%, a melting point of 195° C. and an MFI (372/5) of 100.

EXAMPLE 5

[0071] 25 l of deionized water are charged to a polymerization reactorwith a total volume of 40 l, provided with an impeller stirrer. Thereactor is sealed, and the atmospheric oxygen removed by cycles ofevacuation and nitrogen-flushing, and the vessel is heated to 63° C. 20g of PMPVE, 5 bar of nitrogen and 10 bar of TFE are then fed, so thatthe total pressure of 15.0 bar is achieved. The polymerization isinitiated by pumping in 12 g of APS, dissolved in 100 ml of deionizedwater. As soon as the pressure starts to fall, TFE and PMPVE aresupplemented by introducing the gases in a ratio PMPVE/TFE of 0.004, sothat the total pressure of 15.0 bar is maintained. The heat generated isdissipated by cooling the vessel wall, keeping the temperature constantat 63° C. After a total of 10 kg of TFE has been fed into the reactor,the monomer feed is interrupted, the pressure in the reactor isreleased, and it is flushed several times with N₂. This gives a PTFEproduct which has a PMPVE content of 0.04%, a melting point of 341° C.and an SSG of 2.165.

COMPARATIVE EXAMPLE 5

[0072] The polymerization is carried out as described in example 5, buton this occasion only TFE is utilized. This gives a PTFE product with amelting point of 343° C. and an SSG of 2.16.

COMPARATIVE EXAMPLE 6

[0073] The polymerization is carried out as described in example 5,except that on this occasion PPVE-1 is utilized instead of PMPVE in thefeed supply. This gives a PTFE product which has a PPVE-1 content of0.04%, a melting point of 344° C. and an SSG of 2.165.

[0074] Compared with a PPVE-1-modified PTFE product from comparativeexample 6, the elongations of the inventive product from example 5 areabout 100% lower after 100 h at tensile stresses of 3 MPa, 5 MPa and 7MPa. Compared with an unmodified PTFE product from comparative example5, the elongation is indeed about 400% lower. The inventive productsfrom example 5, PMPVE-modified PTFE products, therefore show markedlylower tendency to deform under load, to exhibit what is known as “coldflow”, than previously known PTFE grades from comparative examples 5 and6. This gives substantial performance advantages, particularly in usesin gasket materials. TABLE 2 (Comparison of elongation after 100 h inPMPVE- and PPVE-1-modified PTFE products, and comparison with anunmodified PTFE) % elongation % elongation % elongation Comonomer 3MPa/100 h 5 MPa/100 h 7 MPa/100 h in % Example 5 0.41 1.29 3.66 0.04Comparative 1.32 4.04 14.55 0 Example 5 Comparative 0.87 2.28 6.36 0.04Example 6

1. A polymer having a melting point of at least 1 60° C., having from 60to 99.99% by weight of units derived from tetrafluoroethylene, from 0 to30% by weight of units derived from hexafluoropropylene, from 0 to 10%by weight of units derived from other olefinic fluorinated ornon-fluorinated monomers, from 0 to 35% by weight of units derived fromperfluoro alkyl vinyl ether having from 1 to 5 carbon atoms in the alkylradical, and from 0.01 to 35% by weight of units derived from one ormore perfluoro vinyl ethers of the formula: CF₂=CFO(CF₂)_(n)OR_(f) wheren is an integer from 2 to 6 and R_(f) is a perfluoroalkyl group havingfrom 1 to 6 carbon atoms:
 2. A polymer according to claim 1 wherein theamount of units derived from tetrafluoroethylene is between 65% byweight and 99.99% by weight.
 3. A polymer according to claim 1 whereinthe amount of units derived from tetrafluoroethylene is between 70% byweight and 99.99% by weight and the amount of units derived from saidperfluorovinyl ethers is between 0.01% by weight and 30% by weight. 4.The polymer as claimed in claim 1, wherein the perfluoro vinyl ether hasthe formula CF₂═CF—O(—CF₂)₂—OCF₃ or CF₂═CF—O(—CF₂)₃—OCF₃.
 5. The polymeras claimed in any of the previous claims, having less than 70 end groupsother than —CF₃ per 106 carbon atoms.
 6. The polymer as claimed in anyof the previous claims in the form of an agglomerate, melt granulate,compacted agglomerate or aqueous dispersion.
 7. Method of making anextruded article comprising extruding a melted composition comprising apolymer having a melting point of at least 160° C., having from 60 to99.99% by weight of units derived from tetrafluoroethylene, from 0 to30% by weight of units derived from hexafluoropropylene, from 0 to 10%by weight of units derived from other olefinic fluorinated ornon-fluorinated monomers, from 0 to 35% by weight of units derived fromperfluoro alkyl vinyl ether having from 1 to 5 carbon atoms in the alkylradical, and from 0.01 to 35% by weight of units derived from one ormore perfluoro vinyl ethers of the formula:CF₂═CF[O(CF₂)_(n)]_(m)(OCF₂)_(x)OR_(f) where n is an integer from 2 to6, m is an integer from 1 to 3, x is an integer from 0 to 4, and R_(f)is a perfluoroalkyl group having from 1 to 6 carbon atoms.
 8. Extrudedarticle comprising a polymer having a melting point of at least 160° C.,having from 60 to 99.99% by weight of units derived fromtetrafluoroethylene, from 0 to 30% by weight of units derived fromhexafluoropropylene, from 0 to 10% by weight of units derived from otherolefinic fluorinated or non-fluorinated monomers, from 0 to 35% byweight of units derived from perfluoro alkyl vinyl ether having from 1to 5 carbon atoms in the alkyl radical, and from 0.01 to 35% by weightof units derived from one or more perfluoro vinyl ethers of the formula:CF₂═CF[O(CF₂)_(n)]_(m)(OCF₂)_(x)OR_(f) where n is an integer from 2 to6, m is an integer from 1 to 3, x is an integer from 0 to 4, and R_(f)is a perfluoroalkyl group having from 1 to 6 carbon atoms.
 9. Extrudedarticle according to claim 7 wherein said article is selected from thegroup consisting of a film, a tube, a hose and a cable having saidpolymer as an insulating medium.
 10. Use of a fluoropolymer having amelting point of at least 315° C., having from 98.5% to 99.99% by weightof units derived from tetrafluoroethylene and from 0.01 to 1.5% byweight of units derived from one or more perfluoro vinyl ethers of theformula: CF₂═CF[O(CF₂)_(n)]_(m)(OCF₂)_(x)OR_(f) where n is an integerfrom 2 to 6, m is an integer from 1 to 3, x is an integer from 0 to 4,and R_(f) is a perfluoroalkyl group having from 1 to 6 carbon atoms; formaking gaskets or an inner lining of pipes or containers.
 11. A methodfor producing a polymer as claimed in claim 1 wherein (i)tetrafluoroethylene, (ii) one or more perfluoro vinyl ethers of theformula: CF₂═CFO(CF₂)_(n)OR_(f) where n is an integer from 2 to 6 andR_(f) is a perfluoroalkyl group having from 1 to 6 carbon atoms, and(iii) optionally one or more further olefinic fluorinated ornon-fluorinated monomers and or a perfluoro alkyl vinyl ether havingfrom 1 to 5 carbon atoms in the alkyl radical, are copolymerized throughaqueous emulsion polymerization or suspension polymerization in theappropriate amounts so as to obtain a polymer as defined in claim 1.